As the Enterprise nears the Pallas 14 system, the cloud engulfs planet Alondra, and the planet disappears from sensors. The cosmic cloud engulfed it, and broke it up into small pieces in seconds. Kirk asks, "Spock, is it possible that the cloud consumes planets?" Spock answers in the affirmative. Suddenly and without warning, the cloud changes course, heading directly for Mantilles. Kirk orders speed increased to warp 8 in order to intercept the cloud more quickly.

"Captain's Log: Supplemental. At warp eight, we will intercept the cloud before it reaches the inhabited planet Mantilles, but we are uncertain what we can do to stop a matter-energy cloud which measures thousands of times more than our vessel."

The Enterprise maneuvers itself directly into the path of the cloud and the ship is engulfed. When Kirk gives the order to fire, Spock reports that the cloud seems to be absorbing the phaser energy.

Since it is unknown if the entity has selective abilities, the Enterprise crew assumes that not only Mantilles, but all planets and star systems are in danger. The Enterprise decides to warn Bob Wesley, governor of Mantilles. But before it can do so, the ship is engulfed by the cloud, and unknown objects comprised of highly charged, gaseous antimatter swarm the vessel, causing a large power drain of the shields. An antimatter charge from the shield is successful in dissipating the objects.

"Captain's Log: Supplemental. The Enterprise has only 15 minutes left before her power is drained and we are left helpless."

Research indicates that the cloud is actually a living being, and that the objects act like enzymes, slowly breaking up the remnants of Alondra and the Enterprise. After more studying of the creature's anatomy, the Enterprise plots a course through the digestive system, with enormous power consumption. To regenerate the ship's antimatter reserves, Montgomery Scott cuts off a piece of the creature's villi, and uses it to create power regeneration of the ship's anti-matter reserves, with minutes to spare. Kirk sighs, "Scotty, you've just given the Enterprise, and Mantilles, a chance to live." Scott replies, "Thank you, sir." Spock then ascertains the being has a brain and, on hearing this, Kirk plans to use the photon torpedoes to destroy the cloud's brain. When Spock begins to remind Kirk of the regulations against the killing of intelligent creatures, Kirk stops him, well aware of the regulations. But they don't yet know if the cloud is intelligent and it is a fact that every living thing on Mantilles will die, unless they stop the cloud, and so if Kirk is forced to make a choice, he chooses saving Mantilles.

"Captain's Log: Stardate 5372.0. Spock and Uhura are using our sensors to prepare a detailed chart, so we may determine targets for our photon torpedoes."

Feeling very unsure about his decision, Kirk asks, "Am I doing the right thing, Bones? Once I said that man rose above primitiveness by vowing, 'I will not kill, today.'" The ever-pragmatic Bones replies, "But you can't let this thing destroy eighty million lives, either." Spock tells them the brain is so vast that the Enterprise's entire complement of armaments may not guarantee its destruction. He determines that they would have to destroy the Enterprise itself and direct the explosion right at the cerebral cortex. Kirk orders Scott to prepare the ship for self-destruct and Spock to try to establish contact with the creature.

"Captain's log, stardate 5372.1. The cloud will reach Mantilles in seven minutes. Spock has been working on the mechanics of reaching the cloud's thoughts, but I doubt if there's any time left. And even if we do establish contact, can we communicate with it and persuade it not to kill?"

"Listen to me. Listen to me."

Spock uses a Vulcan mind meld to contact the creature and allows the creature to see glimpses of Human life for the first time. The creature comes to understand that its food, the planets, are inhabited by "many somethings, so small, but alive. Comprehend. Not desire to consume other beings." The creature allows the Enterprise to leave peacefully and agrees to depart the galaxy. Kirk asks, "Spock, what did you perceive?" The first officer replies, "The wonders of the universe, captain. Incredible, completely incredible."

This episode was written by Marc Daniels, who directed many episodes of Star Trek's original series. Gene Roddenberry encouraged him to develop the idea for this episode. "I have to admit," commented Daniels, "that it was different but fun to do some writing rather than just directing." (Starlog, issue #114, p. 44) This episode was influenced by Daniels having enormous fondness for the premise of one of the episodes he directed, "The Doomsday Machine"; he liked it so much that he transferred many elements in this episode's script from that earlier plot. (These Are the Voyages: TOS Season Two, "The Doomsday Machine")

Though Robert Wesley was played by Barry Russo in the original Star Trek series, James Doohan provided Wesley's voice for this episode.

Some of the shots of Earth, shown to Spock on a computer screen as he is melding with the cloud intelligence, were taken from stock footage of the animated series Lassie's Rescue Rangers, another 1973Filmation TV show. The scenes that came from the Lassie show feature children running with a dog.

LieutenantArex appears for the second time in this episode, and it is the first where he speaks.

Prior to this installment, Robert Wesley appeared in a single episode of TOS: "The Ultimate Computer". In that episode he wore a gold command tunic, but in this one, he can be seen on a viewing screen wearing a blue one.

This episode is where we see, for the first time, the Enterprise's antimatter nacelles, where Kirk and Scotty use the piece of villi extracted from the cloud-creature to recharge the antimatter engines.

Kirk paraphrases himself in this episode, stating, "Man rose above primitiveness by vowing, 'I will not kill, today.'" While this line is never explicitly spoken by Kirk in any previous episode, it does echo the meaning of a speech that Kirk makes in TOS: "A Taste of Armageddon", in which he states, "We're Human beings with the blood of a million savage years on our hands, but we can stop it. We can admit that we're killers, but we're not going to kill, today. That's all it takes. Knowing that we won't kill, today."

The editors of Trek magazine collectively scored this episode 2 out of 5 stars (a rating that they termed "fair"). (The Best of Trek #1, p. 109)

In the unofficial reference bookTrek Navigator: The Ultimate Guide to the Entire Trek Saga (p. 161), co-writer Mark A. Altman rates this episode 3 out of 4 stars (defined as "good") while fellow co-writer Edward Gross ranks the episode 3 and a half out of 4 stars (defined as "great"). Altman characterizes the outing as "one of the most interesting" spatial anomaly episodes and goes on to say, "The animation competently conveys the size and shape of the marauding cloud [....] Spock's mind-meld with the cloud is both logical and satisfying, and Kirk and Spock's wrestling with the need to protect life-forms is intelligent and provocative." Altman also remarks on the episode's links to other canon productions, calling the reuse of the Robert Wesley character "cool continuity!" Gross describes the episode as "highly satisfying" and "one of the best the animated spin-off has to offer," also giving kudos to Marc Daniels and saying that the episode "nicely encapsulates" the philosophy of Star Trek.